1. Field of the Invention
The present invention relates to methods and devices for optimizing hemodynamic variables in implantable cardiac therapy devices (ICTDs). More particularly, the present invention uses a low frequency analysis of various hemodynamic variables to optimize ICTD performance.
2. Background Art
An implantable cardiac therapy device (ICTD) is a medical device that is implanted in a patient to monitor electrical activity of the heart and to deliver appropriate electrical and/or drug therapy, as required. ICTDs include, for example, pacemakers and cardioverter defibrillators.
ICTDs are in electrical communication with the heart. To deliver effective therapy, ICTDs use various forms of physiological information and feedback. Volumetric changes in blood flow and changes in blood pressure, which are closely related, are sources of physiological information that can be used to optimize ICTD performance. Plethysmography is one technique that provides information on volumetric changes in blood flow and changes in blood pressure.
Plethysmography is a generic term referring to a variety of techniques for monitoring volume changes. For example, plethysmography has been used to measure volume changes of the lungs due to respiration, or volume changes in blood in vessels of a limb or tissue segment. When applied to measurements of blood volume in living animals, changes occur in a pulsatile manner with each beat of the heart as blood flows in and out of a portion of the body. The study of vascular activity by fluid displacement methods dates back to at least 1890. Some contemporary techniques include strain gauge, pneumatic, impedance, Doppler, and photoelectric plethysmography. When applied to a body, a plethysmography device produces a waveform that is similar to an arterial pressure waveform as volume changes and pressure changes in fluid are closely related. This waveform is useful in measuring pulse velocity and indicating arterial obstructions.
Plethysmography is a convenient source of information because it can be non-invasive. Furthermore, even when a particular plethysmography technique is invasive, it remains extra-vascular, and thus eliminates various risks associated with intra-vascular measurements such as thrombus, embolus, infections and internal bleeding.
Conventional attempts to use plethysmography to optimize ICTD performance have focused on pulse amplitude. The pulse signal occupies the frequency band of approximately one to ten Hertz (Hz) in the plethysmography signal spectrum. Pulse amplitude is an appealing measure because it directly reflects the contractility and mechanical efficiency of the heart. However, it is sensitive to numerous sources of variability, such as respiration and intrinsic fluctuations in sympathetic outflow (Meyer waves). More importantly, the pulse amplitude can increase even as the mean arterial pressure falls, so that decreases in hemodynamic function can be misinterpreted. It would be beneficial to obtain plethysmographic data that is insensitive to the above sources of variability.